Improvement in the speed and increase in the capacity by wavelength division multiplexing is further progressing in the optical communication system in intermediate and long distances. The current main optical communication system uses wavelength division multiplex communication with determined wavelength channel spacing. Therefore, with 50 GHz spacing in the band of an optical fiber amplifier, approximately 100 channels can be used.
When the channel spacing is Δf[Hz] and the transmission rate is B[bit/s], B/Δf[bit/s/Hz] shall be spectral efficiency. When Δf=50 GHz and the transmission rate is 100 Gbit/s for each channel, the spectral efficiency will be 2 bit/s/Hz.
As the band of the optical fiber amplifier is limited, it is necessary to improve the spectral efficiency. However, simply increasing the bit rate of a signal in order to improve the spectral efficiency raises a problem of interchannel crosstalk. Thus, study on optical multi-level modulation and optical OFDM (Orthogonal Frequency-Division Multiplexing) has been progressing as a next generation optical communication system. The optical multi-level modulation is a method to increase the amount of information by multi-leveling using an amplitude and a phase of a light without increasing the spectral band unlike optical intensity modulation of a related art using two values of 0 and 1. Moreover, in the optical OFDM, an OFDM signal is generated by an electrical signal and is optically modulated, and optical subcarriers are set in an orthogonal state to multiplex. This solves the crosstalk problem, thereby improves the spectral efficiency.
An optical signal with multi-leveling and multiplexing mainly involving electrical signal processing in this way is transmitted and demodulated into an electrical signal by a receiver. An analog-to-digital (A/D) converter is required in the subsequent stage of a PD (photodiode) of an optical demodulation circuit. Currently, an A/D converter using an electrical circuit is commonly used.
On the other hand, there are many proposals for the optical A/D converter that directly derives analog quantity of an optical signal as a digital value because of the feature of rapidity included therein. For example, Patent Literature 1 splits an optical signal by different predetermined split ratios to represent the amount of a light by a predetermined ratio and detects an optical analog quantity of the input optical signal by evaluating whether each split optical signal reaches a threshold.
Further, Patent literature 2 composes in optical A/D conversion means a feedback system through a nonlinear optical element for an input optical signal, which is an analog signal, and thus a first output light, which is a digital signal, is sequentially obtained from the optical A/D conversion means.
In Patent Literature 3, an optical coding circuit optically codes pulse trains of a signal light having a first wavelength according to control light, which is pulse trains of a light analog signal having a nearby second wavelength different from the first wavelength and is optically sampled, by a plurality of optical coders provided with optical nonlinear elements having periodic characteristics whose input and output characteristics regarding optical intensity are different, and outputs the pulse trains of the plurality of optically coded signal lights from each optical coder. Then, an optical quantization circuit outputs pulse trains of a carrier light having a nearby third wavelength that is different from the first wavelength as an optical digital signal after conducting optical threshold processing according to the pulse trains of the plurality of optically coded signal lights and optical quantization using a plurality of optical threshold processing devices provided with optical nonlinear elements which are respectively connected to each optical coder and have periodic input and output characteristics regarding the optical intensity.
Patent Literature 4 is characterized in that a plurality of interference optical modulators are included, a photovoltaic element is formed on the same substrate, and an output voltage of this photovoltaic element is applied on the interference optical modulators. Therefore, in this example, a PD receives an intensity signal light once and converts the intensity signal light into a voltage signal, thereby determining the speed of the entire circuit by the speed of this electrical signal.
Patent Literature 5 discloses a technique to sample a signal light using probe light and perform A/D conversion. Moreover, Patent Literature 6 discloses a technique to successively perform A/D conversion by light subtraction.